Technical Field
The present invention relates to a friction material to be used for disc brake pads, brake linings and clutch facings of automobiles, railway vehicles, industrial machines, etc.
Background Art
A friction material used for brakes such as disc brakes and drum brakes or clutches used in automobiles, etc. is generally formed of materials such as a friction modifier for imparting a friction function and modifying friction performance thereof, a fiber base material having a reinforcing function and a binder for integrating these ingredients to impart strength. The friction material plays a role of changing kinetic energy to thermal energy by frictional engagement with a counterpart material thereof, so that excellent heat resistance, wear resistance, high friction coefficient and stability of the friction coefficient are necessary, and further, it is also required that squeal is less likely to be generated.
For the purpose of improving the strength, the heat resistance and the wear resistance of the friction material and ensuring the friction coefficient during high load, potassium titanate fiber has been conventionally used as the fiber base material. However, the potassium titanate fiber is respirable in size, and the possibility of being cracked into more respirable thin fiber is pointed out. Accordingly, the use thereof is unfavorable on working environmental.
In recent years, therefore, it has been discussed to use a non-fibrous titanate as a substitute for the potassium titanate fiber. For example, Patent Documents 1 and 2 disclose friction materials which can provide the friction function and strength equivalent to those in the conventional ones, without using a whisker-shaped titanate compound.
In addition, as another fiber base material, copper fiber has a function of enhancing fade resistance because it is a material having excellent ductility and high thermal conductivity. The copper fiber is sometimes used as a copper powder in a powdery form. However, in a friction material containing copper or a copper alloy, copper is contained in wear debris generated by braking, and the possibilities of causing pollution of rivers, lakes and oceans, etc. have been pointed out. Therefore, movements to restrict the use thereof have increased.
In recent years, therefore, friction materials having excellent friction function even when the content of a copper component having a high environmental load is small have been developed. For example, Patent Document 3 discloses a friction material not containing copper which shows a friction coefficient and wear resistance equivalent to those of a friction material containing copper by allowing a titanate having a tunnel-like crystal structure and a titanate having a lamellar crystal structure to be contained as titanates. In addition, Patent Document 4 discloses a friction material showing excellent fade resistance regardless of the presence or absence of a copper powder.
Patent Document 1: JP-A-2001-172612
Patent Document 2: JP-A-2007-277418
Patent Document 3: JP-A-2015-147913
Patent Document 4: WO-A1-2016/063688